This invention relates to photoconductive elements and, more particularly, to photoconductive elements of polycrystalline silicon for integrated circuits.
Photoconductive elements (PCEs) can be used to form optoelectronic circuits for coupling a light signal with an electronic circuit. Such coupling can be used for fast response gating or for electrical sampling.
Bulk silicon (Si) is a photoconductive material. Silicon is also a common substrate for integrated circuit fabrication. Thus, optoelectronic circuits can be formed on bulk silicon using standard Si integrated circuit processes. One measure of PCE performance is the response to an input light pulse as measured by the time interval for the response rise and fall, i.e., the full pulse width at half-maximum response (FWHM). Ion beam irradiated Si has generated pulses with 14-20 picosecond (ps) FWHM pulses widths. Representative bulk Si performance is reported in R. B. Hammond et al., "Integrated Picosecond Photoconductors Produced on Bulk Si Substrates," 45 Appl. Phys. Lett., 404 (1984), incorporated herein by reference.
An improved PCE is formed by a polycrystalline Si (poly-Si) layer over an isolating layer of SiO.sub.2. A suitable poly-Si layer is formed using a low pressure chemical vapor deposition (LPCVD) process and an ion beam damage dose of 10.sup.15 cm.sup.-2 of 1.6 MeV Ne. A FWHM pulse width response of 5.5 ps was obtained, which was the circuit limited response rather than the intrinsic limit of the PCE.
Radiation damage to the poly-Si layer is required only through the thickness of the layer rather than the deep damage needed for bulk Si, where long-lived currents are generated deep in the substrate. Thus, lower energy ion implanting devices could be used to enhance process compatibility with other integrated circuit components. A dicussion of early poly-Si PCE performance is provided in R. B. Hammond et al., "On-Chip, Picosecond, Electrical-Characterization Measurements for Si Integrated Circuits," in Ultrashort Pulse Spectroscopy and Applications, M. J. Soileau, Ed., Proceedings of SPIE--The International Society for Optical Engineering, Los Angeles, Calif., Jan. 22-23, 1985, Vol. 533, pp. 136-138. Ion damaged poly-Si had improved pulse width response but with an attendant low signal level which provided a very low signal-to-noise ratio.
This problem is addressed in the present invention and an improved poly-Si PCE is provided with a substantial increase in signal level output.
It is an object of the present invention to provide a fast response PCE with an improved signal-to-noise ratio.
It is another object to provide a fast PCE which can be integrally formed with conventional integrated circuit components.
One other object is to enable the use of commercial implanters in forming a damage profile in the poly-Si layer.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.